This invention relates generally to filters and more particularly to tunable filter circuits.
Many applications require a filter whose corner frequency is variable. These filter circuits are referred to as "tunable" because the corner frequency can be varied or "tuned" responsive to some external variable. An example of what could be considered a tunable filter is shown in FIG. 1. The filter 10 in FIG. 1 has been referred to as an "Active R Filter." The filter 10 includes an operational amplifier (op-amp) 12 and a resistor divider network comprised of resistors RA and RB.
Much analysis has been performed on the frequency response of the basic op-amp. See, e.g., Paul R. Grey and Robert G. Meyer, ANALYSIS AND DESIGN OF ANALOG INTEGRATED CIRCUITS 452-457 (2d ed. 1984) (analyzing the frequency response of the ubiquitous 741 op-amp). It is well known that the frequency response of an op-amp is dominated by an internal compensation capacitor (C.sub.c), which is provided to ensure stability of the op-amp. The compensation capacitor and an equivalent resistance of the transconducance (g.sub.m) stage form an RC time constant which produces a dominant pole in the op-amp.
The "active R filter" 10 uses the resistor divider network at the output of the op-amp to change the effective "R" of the time constant thereby varying the dominant pole of the op-amp. The filter 10, therefore, has a pole that can be "tuned" to a desired corner frequency by the appropriate selection of the resistor values. This approach has the advantage of producing a tunable filter using relatively few components.
There are several problems, however, with the active R filter approach. First, the unity gain frequency of the op-amp is notoriously unpredictable due to process variations in forming the components that comprise the equivalent resistance and the compensation capacitor. Thus, the corner frequency of the filter 10 cannot be varied with any degree of certainty. A second problem is that the unity gain frequency is temperature dependent. Third, the filter is non-linear over a portion of its input operating range. Finally, the filter 10 has only a single pole, neglecting the poles due to the parasitic capacitances of the op-amp 12, which limits its application. While some of these problems can be overcome, to do so requires additional circuitry. Accordingly, a need remains for a tunable filter that does not suffer from the limitations of active R filters.